1. Field of the Invention
The present invention relates in general to sensing liquid levels in containers holding cryogenic liquids. More specifically, the invention uses fiber optic sensors and anemometry to sense liquid levels in cryogenic liquid containers.
2. Description of the Related Art
Measuring liquid levels using temperature measurement techniques can be difficult to make, especially in cryogenic applications. In a closed system, when a cryogenic tank which is partial gas and partial liquid, pre-cooled and at steady state, the temperature difference between gas and liquid is very small, especially near the transition region between gas and liquid. Because of this decreasing temperature difference between the cryogenic liquid and the gas approaching the transition region, achieving high spatial resolution of the liquid level in a container becomes increasingly more challenging with temperature measurements alone.
The current method employed to measure cryogenic liquid levels in such a closed system container uses cryogenic diodes strategically placed serially along a rod/rack. These diodes have a high degree of precision and accuracy and are able to resolve temperatures of about 0.1 C. They are place along a rod at strategic points of interest to indicate whether liquid is present or not.
This approach works, however, the application is limited to the number of diodes able to be mounted onto a rod and, thus, the liquid level resolution is inherently poor. Also, because each diode has associated with it two wires, this makes installation of the device difficult and cumbersome; for example, to measure a 6 foot tall cryogen tank with 2 inch resolution, it would need 36 diodes lay into a rod, and placed into the cryogen tank. However, it would need at least 72 wires to support the multitudes of diodes being implemented, which would be very difficult to create a feed-out for the wires in the limited space ot the cryogen tank, considering also the instrumentation required to interrogate each individual diode sensor. Hence, with the cryogenic diode approach, the application is limited to preselected, sparely placed positions to measure liquid level and is cumbersome to install.
To overcome these problems it has been suggested to use fiber optic sensors to measure liquid levels in tanks. A patent to Chen, U.S. Pat. No. 7,349,600, discloses a fiber optic sensor that uses a special coating that is heated by in-fiber light to monitor liquid level in a container. To activate the special coating, a portion of the light leaks out of the core of the optical fiber and is absorbed by a thermal coating which radiates heat. The spectral response is different when the optic fiber sensor is submerged in liquid versus when the fiber optic sensor is pulled above the liquid level.
However, the inventors herein have found that simply heating the optical fiber sensor and taking the readings as suggested by the Chen patent does not provide sufficient resolution when the liquid temperature and gas temperature in the transition region of a liquid container is similar (such as in a cryogenic liquid tank). Also, using laser light and applying a thermal coating as suggested by the Chen patent can become expensive.
Therefore, it is desired to provide a system and method employing a fiber optic sensor to measure liquid levels in a container that is extremely accurate and inexpensive in the manufacturing of such specialized fiber.